1. Field of the Invention
This invention relates to a sputter coating apparatus, and more particularly, to the structure of a sputtering target and holder assembly used in such an apparatus.
2. Discussion of the Prior Art
Sputtering is a method for applying thin films to substrate materials, such as wafers used in manufacturing microelectronic components. The process occurs in a low-pressure chamber containing a gas which is typically argon. A negative potential is applied to a cathode structure, establishing an electric field, and electrons from the cathode collide with argon atoms, creating ions and exciting a glow discharge. Accelerated by the cathode's negative potential, the ions travel parallel to the electric field lines and impact a target, composed of the coating material, carried on the cathode structure. The kinetic energy of these ions is sufficiently high to dislodge some target surface atoms, which can condense on the substrate to form the film.
One technique, commonly referred to as magnetron sputtering, uses magnetic fields to enhance the ion bombardment of the target. Magnetic means induce a relatively strong field in the vicinity of the target face, with the magnetic field lines oriented generally perpendicular to those of the electric field. Electrons emitted from the target face are influenced by the magnetic field so that their path of notion becomes curved. Thus, the magnetic field traps the electrons near the target. This results in electron-argon collisions close to the target face, thereby maximizing the ion flux bombarding the target. See U.S. Pat. No. 4,472,259 for an illustration of a typical magnetron sputtering device.
A significant result of the ion bombardment is target heating. Ejection of target material is only one result of an ion impacting the target surface, and most of the system input power appears as heat in the target. Sputtering cathode systems are, therefore, sensitive to design changes that call for increased power, thereby causing an increase in target heating.
U.S. Pat. No. 4,855,033 to Hurwitt provides a cathode and target design which attempts to cool a target carried therein during high-power operations. Hurwitt also describes a sputtering target which is designed in an attempt to control its deformation to ensure effective cooling and retention of sputtering face orientation. In addition, the sputtering cathode assembly is described as combining the functions of a magnetic pole piece, a target cooling means, and a mechanical stabilizer for the target:
Specifically, FIG. 1 (PRIOR ART) shows an embodiment of the Hurwitt patent. The cathode assembly 20 includes support structures, a cathode body 23, a target nest 24 and target 26. The assembly is generally cylindrical in form, adapted for the coating of circular substrates such as wafers used in the manufacture of microelectronic components.
The cathode body is formed of a strong ferromagnetic material to enclose and stabilize the holder and target. The cathode body side walls include means for partially enclosing cooling passages, and they may also serve as a magnetic pole piece.
The target nest 24 is located in the central well of the cathode body, and takes the form of a relatively flat, hollow cylinder with a closed back 25 and side walls 27 defining a central cavity. It is composed of heat-conductive material. The back face of the nest conforms to the cathode body. The outer periphery of the holder and central area of its back side include means for partially enclosing coolant passages, the former in registration with cathode body enclosing means, to form coolant channels at the interface between the nest and the body, and other channels in the nest central area each of which is in fluid communication with coolant inlet and outlet means.
Sputtering target 26 is located within the central cavity of the nest, and preferably is an annular disk having a central aperture 53. It is composed of coating material. The target outer rim and the nest inner wall are formed with a helical thread 50, extending the entire vertical length of the wall. This structure permits the target to be screwed into the cavity.
The back face of the sputtering target 62 and the front face of the nest central cavity form complementary surfaces, with the target back face having a convex, arch-like shape, formed as either a convex curved surface or as two surfaces whose intersection encloses a obtuse angle, the apex of which projects outward from the target back face. In one embodiment, one surface is planar, generally normal to the cathode assembly access, and the other is conical. The target back face, therefore, permits a controlled deformation of the target under expansive stresses imposed by heat buildup, such that the target will be urged into forceful, close contact with the cooled surfaces of the holder during operation.
The target is cooled primarily through the side walls 28 of the cathode body, assisted by the nest/target cooling means 50. Also, some cooling exists at an inner rim/retainer convex surface 60.
There are, however, three major problems with the described structure of the Hurwitt patent. Firstly, the threaded target holder and target do not provide much additional cooling since it may often happen that only one surface of the thread is in contact with the cooled cathode. Secondly, it is difficult to install the target because the metal surfaces in contact with each other cannot be lubricated. This is so because sputtering is performed in a high vacuum with high purity targets, and lubricants are not compatible in this environment. Thirdly, after sputtering, it is common to experience some slight distortion of the target, and Hurwitt's design ensures that this distortion is directed towards the outside diameter. Thus, removing the target may become extremely difficult or even impossible without signicant damage to the target holder. For example, it is common for users to remove the target with a chisel and hammer.